(1) Field of the Invention
The present invention relates to an electrostrictive effect element and, more particularly, to a laminated-type electrostrictive effect element and a method of producing the same.
(2) Description of the Related Art
An example of a typical conventional electrostrictive effect element to which the present invention relates is shown in FIG. 1. As shown therein, the electrostrictive effect element has a laminated or stacked structure in which a plurality of internal electrode conductive layers 1 made of silver-palladium alloy and a plurality of electrostrictive ceramic members 2 superimposed alternatingly are formed. A plurality of glass insulation layers 3 are formed on the exposed end surfaces of every other internal electrode conductive layer on the two opposite sides of the laminated structure. An external electrode conductive layer 4 having a strip or belt-like shape and made of sintered-type silver paste is formed on each of the two opposite sides of the laminated structure so as to interconnect the internal electrode conductive layers 1. A pair of lead wires 5 are soldered to the external electrode conductive layers 4. The feature of this electrostrictive effect element resides in the provision of the glass insulation layers 3.
Another structure of such type electrostrictive effect element is one in which the external electrode conductive layer 4 is improved from the above described sintered-type to hardened-type in order to relieve the stress and prevent silver diffusion to the glass insulation layer during the sintering process of the external electrode conductive layer.
Moreover, there is a further conventional electrostrictive effect element as shown in FIG. 2 which is an improvement over the ones shown in FIG. 1. In such electrostrictive effect element, the insulation glass layers 3 as shown in FIG. 1 do not exist. In other words, in this electrostrictive effect element, a plurality of land conductors 6 are formed on, and electrically connected to the end surfaces of every other internal electrode conductive layer 1 exposed on both sides of the laminated structure, followed by welding of a connection wire 7 by means of wire bonding to each of the land conductors 6 for them to be electrically connected on the respective sides, and by soldering thereto a lead wire 5. It should be noted that, in the configuration of FIG. 2, the glass insulation layer 3 as shown in FIG. 1 can be omitted.
In the first of the above explained three conventional electrostrictive effect elements, the external electrode conductive layer 4 is formed by the printing and sintering of a silver-based conductive paste. In this case, the silver will diffuse into the glass insulation layer 3, during the sintering of the conductive paste, since the sintering temperature of the conductive paste (about 600.degree. C.) is close to the sintering temperature of the glass (about 620.degree. C.) used as the glass insulation layer 3. Therefore, the electrical insulation characteristics of the glass insulation layer 3 have a tendency to deteriorate in this type of electrostrictive effect element. This phenomenon is significant especially in highly humid atmosphere and will cause insulation failure during usage in extreme conditions.
The conventional electrostrictive effect element of the second type eliminates the above mentioned disadvantage by making use of a hardening-type conductive paste (hardening temperature: room temperature.about.150.degree. C.) instead of the sintered-type conductive paste as the material for the external electrode conductive layer, whereby it is possible to prevent the silver diffusion into the glass insulation layer by lowering the formation temperature of the external electrode conductive layer 4. In this electrostrictive effect element, the aspect of insulation deterioration phenomenon of the glass insulation layer 3 attributable to silver diffusion may definitely be improved. However, with the hardening-type conductive paste, the resistance is high as compared with that in the sintered-type due to its structure wherein metal particles are not like in a dispersed state within the resin. Thus, in order to have it connected to the internal electrode conductive layer 1, with sufficiently a low resistance, the contact surface with the internal electrode conductive layer 1 must be made fairly large. In the case of an electrostrictive effect element of a laminated-type, the thickness of the internal electrode conductive layer 1 connected with the external electrode conductive layer 4 is less than 10 .mu.m which is very thin. Therefore, the dielectric dissipation factor (tan .delta.), as the element characteristic, will increase as the contact resistance between the external electrode conductive layer 4 and the internal electrode conductive layer 1 become large in the second type conventional electrostrictive effect element where a hardening-type conductive paste is used for the external electrode conductive layer 4. As a result, it will be disadvantageous in pulse signal used applications since the displacement response speed will become slower than that in the first type where the sintered-type conductive paste is used.
Further, in the third type conventional electrostrictive effective element, in order to make an improvement over the first type electrostrictive effect element, the land conductors are used and are interconnected by the connection wires 7 thereby eliminating the need of the glass insulation layer 3. However, with this configuration, the wire 7 will break from fatigue or stress when subjected to repeated drive since a force in the direction of bending will act upon the wire 7 when the element is driven. Also, problems such as short circuiting or breaking of the wire 7 may occur as the wire 7 which is bent can easily come in contact with an opposing electrode or the wire 7 may get caught when it is in a naked state thereby exposing itself from the surface of the element.